Liquid toner compositions for use in electrostatic printing typically contain resin particles within a carrier liquid. In recent years, there has been a trend toward using resins that are more durable when printed. The present inventors have been investigating certain resins, particularly those with a higher acidity than those typically used in the prior art, in an effort to improve the durability of the inks once printed. Certain resins have been found to be particularly effective in producing a durable ink, but the present inventors have identified certain difficulties associated with them, for example in the electrostatic printing processes. Electrostatic printing will briefly be described.
In electrostatic printing systems, it is common practice to develop a hardcopy of an image by using a photoconductive surface. The photoconductive surface is typically on a cylinder and is often termed a photo imaging plate (PIP). The photoconductive surface is selectively charged with a latent electrostatic image having image and background areas. For example, an electrostatic ink composition comprising charged toner particles in a carrier liquid can be brought into contact with the selectively charged photoconductive surface. The charged toner particles adhere to the image areas of the latent image while the background areas remain clean. The image is then transferred to a print substrate (e.g. paper) directly or, more commonly, by being first transferred to a heated intermediate transfer member (often a flexible member, sometimes termed a blanket), and then to the print substrate. Variations of this method utilize different ways for forming the electrostatic latent image on a photoreceptor or on a dielectric material.
Typically, an electrostatic ink composition comprises a thermoplastic resin as the basis for the toner particles, and a non-polar liquid as a carrier liquid in which the toner particles are dispersed. Generally, the toner particles contain a colorant such as a pigment. A charge director, also called charge control agent or imaging agent, is also added to the dispersion to induce charge on the particles.
The present inventors have found that with certain resins, particularly those with a relatively high acidity (particularly an acidity of 80 mg KOH/g or more, especially an acidity of 100 mg KOH/g or more), the toner particles do not transfer as effectively as desired from the imaging plate to the intermediate transfer member, and then from the intermediate transfer member to a print substrate, such as paper.
It would be desirable to provide a liquid toner composition that avoids or mitigates at least one of the transfer problems associated with the toner compositions described above. It would also be desirable to provide a liquid toner composition that produces a durable ink once printed, while also avoiding or mitigating at least one of transfer problems associated with the toner compositions mentioned above.